Subsea heat exchanger

ABSTRACT

A subsea heat exchanger comprising a bundle of tubes comprising at least one tube winding arranged to operate submerged in water and effective for guiding a fluid to be cooled by surrounding water in contact with the tube, the bundle of tubes defining a longitudinal extension and a perimeter. A plurality of nozzles are distributed in spatial relation to said perimeter, wherein the nozzles are effective for discharging jets of water impinging on the tubes, the nozzles oriented to induce, in the ambient water volume, a displacement that passes the perimeter at a plurality of locations and directions.

TECHNICAL FIELD

The present invention relates to forced convection heat exchangers for subsea use.

BACKGROUND OF THE INVENTION

In the recovery and production of gas and oil from subsea wells heat exchangers are frequently required for example to control the temperature of the production fluid or cooling media used in production equipment.

Subsea heat exchangers are often based on natural convection to seawater, and may be rated in passive or forced convection coolers. Basically, a passive convection cooler comprises a series of tubes which are exposed to seawater that is allowed to circulate freely between the tubes. Passive convection coolers are usually bulky and heavy structures and suffer from uncontrollable operation parameters such as variations in seawater currents and temperature, resulting in little or no control over the cooling process.

A forced convection cooler typically comprises a bundle of tubes enclosed by an outer shell or duct which is associated with a driven pump or propeller that generates a forced flow of water/seawater through the duct. Examples of forced convection coolers may be found in international patent application publication nos. WO2010/002272A1, WO2012/141599A1 or WO2013/004277A1. Ducted, forced convection coolers provide enhanced control over the temperature of the target fluid but may still suffer from biological fouling and deposition of material on heat exchanger tubes.

SUMMARY OF THE INVENTION

The present invention aims at providing a forced convection heat exchanger with improved control over the cooling process.

Another object of the present invention is to provide a forced convection heat exchanger which is less susceptible of biological or particulate fouling of heat exchanger tubes.

Yet another object of the present invention is to provide efficient cooling by forced convection in a heat exchanger having compact design.

These and other objects are met in a heat exchanger according to the present invention wherein a turbulent flow of water is created across the heat exchanger tubes by means of a plurality of nozzles arranged to discharge jets of water towards the tubes.

More particularly, a subsea heat exchanger is provided having a bundle of tubes comprising at least one tube winding arranged to operate submerged in water and effective for guiding a fluid to be cooled by surrounding water in contact with the tube, the bundle of tubes having a longitudinal extension and a perimeter, and a plurality of nozzles distributed in spatial relation to the parameter, wherein the nozzles are effective for discharge of jets of water impinging on the tubes, the nozzles oriented to induce in the ambient water volume a displacement that passes the perimeter at a plurality of locations and directions.

The nozzles are configured to generate turbulent displacement of water near the tubes.

A turbulent flow can be achieved when, according to an embodiment, nozzles are arranged at an angle in a plane transversally to a longitudinal extension of the tube bundle to discharge jets of water at from about tangential direction to 90° angle of impact with the perimeter of the bundle of tubes.

In this respect, the invention can be realized in different embodiments and configurations with respect to the disposition of nozzles in relation to the bundle of tubes.

In one embodiment the nozzles are arranged radially outside the bundle of tubes. More precisely, the nozzles can be arranged radially outside the circular parameter of a helical bundle of tubes, the nozzles mouthing inwards towards a center of the bundle of tubes. In this embodiment the nozzles may be directed to generate, in seawater surrounding the heat exchanger, an inwardly directed displacement which ranges from substantially tangential to the perimeter or substantially radial with respect to the center of the helical bundle of tubes, as seen in a radial plane of the heat exchanger.

In another embodiment the nozzles are arranged inside the bundle of tubes. More precisely, the nozzles can be arranged radially inside a helical bundle of tubes, the nozzles mouthing outwards towards a perimeter of the bundle of tubes. In this embodiment the nozzles may be directed to generate, in seawater within the helical tube bundle, an outwardly directed displacement which may be radial or non-radial with respect to the radial plane of the bundle of tubes.

The angled orientation of nozzles may be utilized to create a rotating movement and displacement of ambient seawater in and about the bundle of tubes.

In each of the above embodiments the nozzles may further be arranged at an inclination in order to generate or support a rising displacement of seawater through the heat exchanger. Thus, alternatively or in addition to the angled orientation of nozzles in radial planes, the nozzles may be arranged at an inclination to the longitudinal axis, in axial plane, to discharge jets of seawater at from about 30° to 90° angle of impact with the perimeter of the bundle of tubes.

The nozzles may be arranged in sets, wherein the nozzles are commonly supplied seawater from a subsea motor and pump assembly which discharges seawater at elevated pressure and flow rate into a manifold. In some embodiments, a manifold supplies seawater to a number of riser pipes extending from the manifold in the longitudinal direction of the bundle of tubes, each riser pipe carrying a set of nozzles, respectively.

Seawater is supplied to the nozzles by means of a subsea motor and pump assembly. By regulating the output from the motor and pump assembly and/or shutting on/off nozzles by means of valves, an active control of temperature in the target fluid is obtainable. To this purpose, a variable speed drive (VSD)-motor driving the seawater pump permits common control of the nozzles.

The nozzles may alternatively be controllable in common through a pressure regulating device in the water distribution manifold.

A set of nozzles may additionally be controllable separately from other sets of nozzles. For instance, the sets of nozzles may be controlled for intermittent discharge of jets of water at an alternating schedule and in consecutive order. This embodiment effectively reduces the capacity which is required by the motor and pump assembly.

In one embodiment pulsating jets of water from the nozzles is obtainable by the installation of a flow pulse generator upstream of the nozzles.

The nozzles may be realized as orifices formed on the riser pipes which are supplied seawater via the manifold.

The nozzles may alternatively be realized as Venturi tubes or ejectors operating in accordance with Bernoulli's principle.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be further explained below with reference to the accompanying, schematic drawings. In the drawings:

FIG. 1 is a side view showing a first embodiment of a heat exchanger according to the present invention;

FIG. 2 is a top view of the heat exchanger of FIG. 1;

FIG. 3 is a top view corresponding to FIG. 2, showing a second embodiment of the heat exchanger;

FIG. 4 is a broken away detail on larger scale showing nozzles arrangement in the heat exchanger;

FIG. 5 is a corresponding detail showing alternative arrangement of nozzles in the heat exchanger;

FIG. 6 is a side view showing another embodiment of the heat exchanger; and

FIG. 7 is a top view showing yet another embodiment of the heat exchanger.

DETAILED DESCRIPTION

It serves to point out that while the invention is described herein with reference to a vertically oriented heat exchanger open to the ambient sea, the teachings presented may likewise be applied in submerged heat exchangers of horizontal or inclined orientations. Accordingly, any term used in the description to define spatial relations shall be understood to include equivalent terms applicable to heat exchangers of other main orientations than the illustrated vertical one.

A forced convection heat exchanger 1 for subsea use utilizes a bundle of tubes, comprising at least one tube winding 2, through which a fluid stream F is advanced under transfer of heat via the tube wall to surrounding seawater SW. The heat exchanger 1 thus operates submerged in seawater, as illustrated in FIG. 1.

The fluid stream F may be a hydrocarbon production fluid stream which requires cooling before reaching downstream located equipment such as pumps or compressors, etc. The heat exchanger 1 is however not limited to the cooling of production fluid but may likewise serve for cooling other fluids involved in subsea hydrocarbon production such as, for example, coolant, lubricant, or barrier fluid.

The heat exchanger 1 further comprises a plurality of nozzles 3 which are distributed in spatial relation to a perimeter O of the bundle of tubes 2. In this context, as used in the disclosure, “spatial relation to the perimeter” shall be understood to mean that the nozzles 3 are distributed along the perimeter O, or at least along substantial portions of the perimeter, and externally or internally at a distance from the perimeter O, as seen in a side view or in a top or bottom view of the heat exchanger.

The nozzles 3 are effective for discharge of jets of seawater towards the bundle of tubes 2. The nozzles 3 are arranged on riser pipes 4, such that each riser pipe 4 carries a set of nozzles 3. The riser pipes 4 are supplied seawater via a manifold 5 which distributes seawater at elevated pressure and flow rate generated by a subsea motor 6 and pump 7.

The operation of the motor and pump assembly 6, 7 may be controlled via a variable speed drive (VSD) 8 and a heat exchanger control unit (HXC) 9 which adjusts the supply of seawater to the nozzles 3 in response to the temperature of the target fluid F, detected by a temperature sensor (TS) 10. In this configuration, the HXC and VSD adjust the operation of the nozzles in common in response to a required cooling effect and reduction of temperature in the target fluid F.

The operation of the nozzles 3 may additionally or alternatively be controlled through valves 11 arranged to permit or cut the flow of water through the riser pipes 4, thus controlling the operation of a set of nozzles 3 separately from the other sets of nozzles.

The valves 11 can be realized as on/off valves, and controlled from the HXC. By opening the valves 11 one at a time in consecutive order, for example, a pulsating discharge of jets towards the bundle of tubes 2 is obtainable. This arrangement also reduces considerably the capacity required by any motor and pump installed to supply seawater to the nozzles. Pulsating jets may alternatively be generated by means of a pulse generator installed in the supply of seawater upstream of the nozzles (not shown in the drawings).

Operational control of the plurality of nozzles may alternatively be achieved from a pressure regulating device 11′ arranged to adjust the flow in the seawater distributing manifold 5, as illustrated in FIG. 1.

In a structurally non-complex way the nozzles may be realized as orifices made through the wall of the riser pipes.

More efficient jets of water may be generated from nozzles made in the form of Venturi tubes or ejectors that operate in accordance with the well-known Bernoulli's principle. A corresponding ejector is shown in FIG. 4; this ejector comprising a nozzle 3 which is installed in a Venturi tube 12 having a diffuser section 13 of increased radius. The nozzle 3 communicates with the riser pipe 4 via a passage 14 through the wall of the riser pipe. A high velocity jet J is discharged from the convergent mouth of the nozzle 3, creating within the tube 12 a low pressure zone that draws in seawater via an open inlet to the tube. The entrained seawater is mixed with the jet in the tube, the mixed flows then being discharged from the discharge end facing the bundle of tubes 2.

The nozzles 3 are oriented for discharge of jets of water that impinge on the heat exchanger tubes 2 inducing, in the ambient volume of water, a flow or displacement R that passes the perimeter O of the heat exchanger tubes 2. The jets of water are split by the tubes and rejoined in zones of turbulent water T on the lee-side of the tubes, substantially as illustrated schematically in FIG. 4. The impinging jets on the windward side of tubes and the turbulent flow on the lee-side of the tubes both contribute to reduce fouling, such as fouling in the form of particle deposition, scaling and biological growth on the heat exchanger tubes.

Whereas the partial view of FIG. 4 illustrates nozzles 3 which are oriented transversally or at right angles to the perimeter O, the partial view of FIG. 5, on the other hand, illustrates nozzles which are angled relative to the perimeter and, more precisely, which are inclined upwards at an angle a relative to the perimeter O. The inclined orientation of nozzles 3 may be used to enhance a rising displacement U of seawater through the bundle of tubes 2, adding to the natural upward displacement in the form of convection currents due to heat which is absorbed by the ambient water. It is conceived that the angle α may vary from about 30° to 90° in practice.

Returning to the embodiment depicted in FIGS. 1 and 2, a heat exchanger design is illustrated wherein a plurality of nozzles 3 are arranged along the circular perimeter of a helical bundle of tubes 2, and radially outside the same. More precisely, a number of riser pipes 4 are distributed about the bundle of tubes, the pipes 4 rising from a circular manifold 5 connecting the riser pipes with the motor and pump assembly 6, 7 (shown in FIG. 1 only). In an embodiment, the riser pipes may be equally angularly spaced about the bundle of tubes 2 as illustrated.

Each riser pipe 4 carries a set of nozzles 3 ¹ to 3 ⁶. Whereas the nozzles 3 ¹-3 ³ on the left hand side of the drawing in FIG. 2 are oriented in radial directions towards the center C of the tube bundle, the nozzles 3 ⁴-3 ⁶ on the right hand side of the drawing are oriented substantially in tangential direction relative to the perimeter of the bundle of tubes 2. Arranging the nozzles at different orientations in the same heat exchanger as illustrated in FIG. 2 is possible. This option is shown herein for illustrating purpose, and it is assumed, that a more frequent practice will involve nozzles which are equally oriented with respect to the perimeter or to the center of the heat exchanger. However, different combinations of number of nozzles and orientations may be conceivable.

It will be appreciated that the external location of the nozzles 3 ¹-3 ³ in FIG. 2 will induce, in the ambient water volume, a displacement wherein the main component of direction is radial towards the center of the bundle of tubes 2. It is likewise appreciable that the external location of the tangentially oriented nozzles 3 ⁴-3 ⁶ in FIG. 2 will induce, in the ambient water volume, a displacement comprising a tangential component of direction, which is deflected towards the interior of the heat exchanger by the jet discharged from the adjacent downstream nozzle. In both cases the externally located and inwardly or substantially tangentially opening nozzles 3 ¹ to 3 ⁶ will generate a displacement R of ambient water which passes the perimeter O of the tube bundle at a plurality of locations and directions, causing turbulence near the tubes. The displacement of water from the exterior to the interior of the bundle of tubes will further cause an axial, upward displacement and replacement of the volume of water that is surrounded by the tube winding 2. In addition the non-radially or substantially tangentially oriented nozzles 3 ⁴-3 ⁶ will generate a rotating displacement of water surrounding the heat exchanger tubes 2 (i.e., a clockwise rotation according to the setup of FIG. 2).

An inverted design of the heat exchanger is illustrated in FIG. 3. The embodiment of FIG. 3 differs from the previous one in that the riser pipes 4 and nozzles 3 are located along a circular perimeter of a helical bundle of tubes 2 and radially inside the same, the nozzles opening outwardly toward the perimeter of the bundle of tubes. Whereas the nozzles 3 ¹-3 ³ on the right hand side of the drawing are oriented substantially in radial direction from the center C, the nozzles 3 ⁴-3 ⁶ on the left hand side of the drawing are oriented in non-radial directions relative to the center C. Again, arranging the nozzles at different orientations in the same heat exchanger as illustrated in FIG. 3 is possible. This option is shown herein for illustrating purpose and it is assumed that a more frequent practice will involve nozzles which are equally oriented with respect to the perimeter or to the center of the heat exchanger. However, different combinations of number of nozzles and orientations may be conceivable.

It will be appreciated that the internal location of the nozzles 3 ¹-3 ³ in FIG. 3 will induce, in the ambient water volume surrounded by the tube winding 2, a displacement wherein the main component of direction is radially outwards with respect to the center C. It is likewise appreciable that the internal location of the nozzles 3 ⁴-3 ⁶ in FIG. 3 will induce, in the ambient water volume, a displacement comprising a tangential component of direction.

In both cases the internally located and outwardly radially or non-radially opening nozzles 3 ¹ to 3 ⁶ will generate in the ambient water volume a displacement R which passes the perimeter O at a plurality of locations and directions, causing turbulence near the tubes. The displacement of water from the interior to the exterior of the bundle of tubes will further cause a replacement (e.g. from below) of the volume of water that is displaced from inside of the tube winding 2. In addition, the non-radially oriented nozzles 3 ⁴-3 ⁶ may generate a rotating displacement of water surrounding the heat exchanger tubes 2 (i.e., an anti-clockwise rotation according to the setup of FIG. 3).

The invention is not limited to any specific number of riser pipes 4, number of nozzles 3 or number of turns in a tube winding 2. Other designs beside the circular configuration illustrated in FIGS. 1-5 are also possible.

An alternative design is illustrated in FIG. 6, showing in a side view a heat exchanger comprising a flat or substantially flat serpentine tube winding 2. A plurality of nozzles 3 are distributed along major portions of a rectangular perimeter O and externally thereto, the nozzles affecting in operation a displacement R of ambient water which passes the perimeter O at a plurality of locations and directions, causing turbulence near the tubes 2.

Yet another alternative design is illustrated in FIG. 7, showing in a top view the uppermost tube layer of a heat exchanger comprising a bundle of tubes including several flattened or substantially flat helical or serpentine tube windings 2. A plurality of nozzles 3 are distributed along major portions of a rectangular perimeter O and externally thereto, the nozzles being arranged in sets on pipes 4 that rise from a manifold 5. It shall be noted that in the top view of FIG. 7 only the uppermost nozzle 3 in each set of nozzles is shown.

Other nozzle designs than the illustrated Venturi tube is feasible, such as a fluidic nozzle designed to generate a self-oscillating jet. Self-oscillating jet nozzles involve no moving parts and require basically no maintenance, which makes them attractive for subsea use and for implementation in the forced convection heat exchanger. The use of nozzles that create self-oscillating jets results in higher heat transfer coefficient and bigger coverage area, thus are required fewer in number compared to standard jet nozzles.

Another group of nozzles, which can be contemplated for use in submerged forced convection heat exchangers, are the nozzles designed to create synthetic jets formed by ambient water through the periodic suction and ejection of fluid out of an orifice to a cavity. A diaphragm which is built into a wall of the cavity is brought into a time periodic motion driven, for example, by a piezoelectric generator or by an electromagnetically driven piston.

Based on the teachings provided herein, other modifications will be possible without departing from the scope of the invention as defined by the accompanying claims.

This written description uses examples to disclose the invention, including the preferred embodiments, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of an embodiment of the present invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 

What is claimed is:
 1. A subsea heat exchanger comprising: a bundle of tubes, comprising at least one tube winding configured to operate submerged in water and to guide a fluid to be cooled by surrounding water in contact with the bundle of tubes, the bundle of tubes comprising a longitudinal extension and a perimeter; and a plurality of nozzles distributed in spatial relation to the perimeter, wherein the nozzles are configured to discharge jets of water impinging on the bundle of tubes, and to induce in an ambient water volume a displacement that passes the perimeter at a plurality of locations and directions.
 2. The heat exchanger of claim 1, wherein the nozzles are further configured to generate a turbulent displacement of water near the bundle of tubes.
 3. The heat exchanger of claim 1, wherein the nozzles are arranged in a plane transversally to the longitudinal extension to discharge the jets of water at from about a tangential direction to about a 90° angle of impact with the perimeter of the bundle of tubes.
 4. The heat exchanger of claim 1, wherein the nozzles are arranged at an inclination in a plane parallel to the longitudinal extension to discharge the jets of water at from about a 30° to about a 90° angle of impact with the perimeter of the bundle of tubes.
 5. The heat exchanger of claim 1, wherein the nozzles are arranged radially outside the perimeter of a helical bundle of tubes, the nozzles mouthing inwards towards a center of the bundle of tubes.
 6. The heat exchanger of claim 1, wherein the nozzles are arranged radially inside a helical bundle of tubes, the nozzles mouthing outwards towards the perimeter of the bundle of tubes.
 7. The heat exchanger of claim 1, further comprising a manifold and a number of riser pipes extending from the manifold in the longitudinal direction of the bundle of tubes, each riser pipe supporting a set of the nozzles.
 8. The heat exchanger of claim 1, wherein the nozzles are controllable in common through a variable speed drive motor driving a sea water pump.
 9. The heat exchanger of claim 1, wherein the nozzles are controllable in common through a pressure regulating device in a water distribution manifold.
 10. The heat exchanger of claim 9, wherein the nozzles are distributed such that a first set of the nozzles is controllable separately from a second set of the nozzles.
 11. The heat exchanger of claim 9, wherein sets of the nozzles are alternatingly operable in consecutive order.
 12. The heat exchanger of claim 1, wherein the nozzles comprise ejectors operating in accordance with Bernoulli's principle.
 13. The heat exchanger of claim 1, wherein a flow pulse generator is installed upstream of the nozzles.
 14. The heat exchanger of claim 1, further comprising: a vertically oriented, non-ducted, spiral wound heat exchanger tubing open to surrounding seawater; a circular manifold arranged in a lower end of the heat exchanger, the manifold supplied with seawater from a motor and pump assembly; a number of riser pipes rising vertically from the circular manifold, angularly spaced about the spiral wound heat exchanger tubing; and a set of the nozzles on each riser pipe, the nozzles mouthing radially inwards towards the center of the spiral wound heat exchanger tubing.
 15. The heat exchanger of claim 1, further comprising: a vertically oriented, non-ducted, spiral wound heat exchanger tubing open to surrounding seawater; a circular manifold arranged in a lower end of the heat exchanger, the manifold supplied with seawater from a motor and pump assembly; a number of riser pipes rising vertically from the circular manifold, angularly spaced inside the spiral wound heat exchanger tubing; and a set of the nozzles on each riser pipe, the nozzles mouthing radially outwards towards the periphery of the spiral wound heat exchanger tubing. 